1. The Field of the Invention
The invention relates to devices, methods, and systems for field emission oscillators and modulators. Specifically, the invention relates to devices, methods, and systems for high-frequency energy coupling in radiation-assisted field emission.
2. The Relevant Art
The increasing performance demands of high-speed computing and communications require the generation of electromagnetic signals at ever-higher frequencies. High-frequency signals are needed to exploit opportunities for higher-speed processing and data transmission. High-frequency signals are also essential for many new applications such as imaging and spectroscopy for identification of molecules in chemical and biological agents or communications signals capable of propagating through highly ionized gases.
Yet the physical constraints of materials and electromagnetic radiation have limited the generation of switchable, tunable signals at frequencies of one terahertz and above. The high-frequency characteristics of vacuum tubes are limited by physical scaling and metallic loses. The high-frequency characteristics of semiconductor-based electronic devices are limited by resistive loses, reactive parasitics, and carrier transit delays. These limitations result in sharp power roll-offs above 1 Terahertz.
The operating frequencies of electronic devices have been increased by taking advantage of the higher switching speeds of optoelectronic devices. The Auston Switch uses pulsed lasers to modulate the conductivity of a photoconductive substrate such as Gallium Arsenide (GaAs). The laser pulse excites electrons from a valence band to a conduction band, changing the substrate from an insulator to a conductor. Auston Switches have switching times of about 500 fs, allowing them to generate extremely narrow electrical pulses or high-frequency signals.
Lasers have also been used to modulate the current in field emission or Fowler-Nordheim tunneling. In field emission, an applied electric field reduces the potential barrier at the surface of a metal or semiconductor. When the potential barrier is reduced to be near the Fermi level of the electrons, the electrons “tunnel” from the metal or semiconductor. The tunneling electrons create an electric current.
A laser pulse can modulate the tunneling of electrons. The response time of field emission to a laser pulse can be as brief as 2 fs, less than one per cent of the response time of the photoconductive substrate in an Auston Switch. Laser-modulated field emission-based devices could be used for high-frequency switching and signal generation. For example, two lasers of different frequencies may excite a tunneling current that oscillates at the difference of the laser frequencies.
In a radiation-assisted field emission device, one or more lasers radiate to an emitting surface, producing a tunneling electron current. The tunneling electron current oscillates or switches at extremely high frequencies. Radiation assisted field emission devices are capable of producing extremely high-frequency signals with high frequency agility, the ability to rapidly change the output frequency. However, the high-frequency response pertains only to the current emitted from the apex of an emitting tip. The high-frequency energy must be effectively coupled for field emission devices to have practical application as switches or signal generators.
U.S. Pat. No. 6,153,872 teaches three techniques for coupling high-frequency energy from the apex of a field-emitting tip. U.S. Pat. No. 6,153,872 is incorporated herein by reference. The techniques include: coating the metal emitting tip with a dielectric so that a Goubau wave may propagate energy along the tip to a load; using a Sommerfeld wave to excite a dielectric waveguide to carry energy to a load; and forming a traveling-wave antenna to radiate energy to a second antenna connected to a load. Although the techniques of U.S. Pat. No. 6,153,872 are partially effective, additional enhancements are required for practical application to laser-modulated field emission devices.
Nanoscale field emission tubes have been built and field emitter arrays with as many as 1010 tips per square centimeter are now used in flat panel displays. Miniature multifunction field emission devices could be built if energy could be efficiently transmitted from field emissions. What is needed is an improvement to the energy coupling from field emission devices, to increase the useful energy from radiation-assisted field emission devices. Improved energy coupling will support the creation of practical terahertz sources.